Ornithine is converted into arginine in the urea cycle. Intermediaries in the ornithine biosynthesis pathway are important in other steps of this cycle. Amino acid N-acetyl transferase (EC 2.3.1.1) catalyzes the first reaction in a pathway that leads to the synthesis of ornithine from L-glutamate giving N-acetylglutamate as its intermediary product. Amino acid N-acetyl transferase is also known as N-acetylglutamate synthetase (AGS) and is encoded by the argA locus in bacteria. No plant N-acetylglutamate has been described to date.
Carbamoyl phosphate synthase I, the mitochondrial enzyme that catalyzes the first committed step of the urea cycle, is allosterically activated by N-acetyl glutamate. The rate of urea production by the liver is, in fact, correlated with the N-acetylglutamate concentration. Increased urea synthesis is required when amino acid breakdown rates increase, generating excess nitrogen that must be extracted. Increase in these breakdown rates are signaled by an increase in glutamate concentration through transamination reaction. This situation, in turn, causes an increase in N-acetylglutamate synthesis, stimulating carbamoyl phosphate synthetase and the entire urea cycle.
Ornithine oxo-acid transaminase (EC 2.6.1.13), also called omithine aminotransferase, catalyzes the conversion of omithine to L-glutamate semialdehyde. The gene encoding ornithine oxo-acid transaminase has been cloned from Aspergillus nidulans, Saccharomyces cerevisiae, Plasmodiumfalciparum, Vigna aconitifolia, rat, mouse and man. No plant sequences encoding omithine oxo-acid transaminase have been described to date.